ca failure in ed polypropylene capacitors
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Failure in metallised polypropylene capacitors
Metallised polypropylene capacitors are used extensively in fluorescent light fittings and on electric
motors. The vast majority last for years without problem, but there are occasional incidents where
they burst or even catch fire.
These types of capacitor possess a safety mechanism called self healing. If an electrical breakdown occurs in
the film, the charges on the capacitor electrodes in the vicinity of the fault flow to that point and are discharge
Considerable energy can be dissipated in the discharge.
This is usually limited by using a
metal film that is so thin that the
metal evaporates due to the high
local current densities. The
evaporation disconnects the fault
from the rest of the film, so that
the effects are limited. The eventtypically lasts for microseconds.
Part of the film is lost and the
capacitance is reduced, but the
capacitor continues to be usable.
A piece of film with a self-healing
spot is shown Figure 1.
Figure 1 SEM image of a group of breakdown points in a polypropylene plast
film at high magnification
Part of the design process for the capacitors consists of balancing the need for film thin enough for self-healing
against the need to keep the series resistance low. After completing the manufacturing steps a selection of the
capacitors are put on life tests to ensure that the weakest spots in the film are disconnected before the
capacitors are put into service. The metal electrodes are sometimes composed of small islands of metal
connected by narrow links. The links are easily broken by an excess current at a breakdown without disturbing
the neighbouring islands.
The energy dissipated in the breakdown appears as
heat and the temperature rises locally. The plastic
film at the breakdown point is carbonised with the
release of gas. If a series of breakdowns occur closetogether within the capacitor or closely spaced in
time, the capacitor may not be able to dissipate the
heat generated in the breakdown events. The
temperature then rises locally with each breakdown
and the gas accumulates. If sufficient breakdowns
occur, the temperature rises high enough to melt th
plastic film.
Figure 2 Optical photograph of a burst
polypropylene capacitor
Then larger breakdowns can occur and significant amounts of gas are generated. The gas can blow out the sid
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of the case or blow the case off entirely. Under extreme conditions the gas mixture can ignite. An example is
shown in Figure 2. These blow-outs can damage neighbouring components.
A simplified thermal model has been used to estimate the temperature rise around a breakdown results of whi
are shown in Figure 3. There is a sharp peak which spreads out as time progresses. If another breakdown puls
occurs within milliseconds or tens of milliseconds and within tens of microns of the first one, the effects will ad
and the temperature will rise even higher. A sequence of similar pulses will raise the temperature hundreds ofdegrees centigrade. The external damage can be prevented but the additional complications results in extra
cost.
Fig.3. A set of plots of the temperature as a function of distance from a breakdown in the form of a disc for a
series of times after the event using a simplified thermal model.
Further information
Other common forms of failure in metallised film capacitors are partial discharges and ion migration to damag
sites. ERA has examined many failed capacitors and batches of capacitors before and after use to assess the
quality of construction. The pattern of the damage on the film can used to assess the processes involved in the
failure. Sometimes it is found that there are batches of suspect capacitors. To find out more call us on +44
(0)1372 367444 or email.
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